The statements in this section merely provide background information related to the present disclosure. Accordingly, such statements are not intended to constitute an admission of prior art.
Spark-ignition (SI) engines introduce an air/fuel mixture into each cylinder that is compressed in a compression stroke and ignited by a spark plug. Compression-ignition (CI) engines inject pressurized fuel into a combustion cylinder near top-dead-center (TDC) of the compression stroke that ignites upon injection. Combustion for both SI engines and CI engines involves premixed or diffusion flames controlled by fluid mechanics.
SI engines may operate in a variety of different combustion modes, including a homogeneous SI combustion mode and a stratified-charge SI combustion mode. SI engines may be configured to operate in a homogeneous-charge compression-ignition (HCCI) combustion mode, also referred to as controlled auto-ignition combustion, under predetermined speed/load operating conditions. The HCCI combustion mode includes a distributed, flameless, auto-ignition combustion process that is controlled by oxidation chemistry. An engine operating in the HCCI combustion mode has a cylinder charge that is preferably homogeneous in composition, temperature, and residual exhaust gases at intake valve closing time. HCCI combustion is a distributed kinetically-controlled combustion process with the engine operating at a dilute air/fuel mixture, i.e., lean of a stoichiometric air/fuel point, with relatively low peak combustion temperatures, resulting in low NOx emissions. The homogeneous air/fuel mixture minimizes occurrences of rich in-cylinder combustion zones that form smoke and particulate emissions.
Engine airflow may be controlled by selectively adjusting position of the throttle valve and openings and closings of intake and exhaust valves. On engine systems so equipped, openings and closings of the intake and exhaust valves may be adjusted using a variable valve actuation system that includes variable cam phasing and a selectable multi-step valve lift, e.g., multiple-step cam lobes that provide two or more valve lift positions. In contrast to the throttle position change, the change in valve position of the multi-step valve lift mechanism is a discrete step change.
When an engine operates in a HCCI combustion mode, the engine operates at a lean or stoichiometric air/fuel ratio with the throttle wide open to minimize engine pumping losses. When the engine operates in the SI combustion mode, the engine operates in stoichiometric air/fuel ratio, with the throttle valve controlled over a range of positions from 0% to 100% of the wide-open position to control intake airflow to achieve the stoichiometric air/fuel ratio.
Engines operating in auto-ignition combustion modes account for operating conditions using calibration tables as part of an overall engine control scheme executed in an engine control module. Known HCCI engine control schemes include calibrations for controlling engine parameters based on a limited number of input parameters including, e.g., engine load, engine speed and engine coolant temperature. Measured output parameters are used to control the amount of hot residuals (via variable cam phasing) and the amount of cold residuals (via exhaust gas recirculation rate) and therefore control in-cylinder gas temperature.
Combustion phasing is an engine operating parameter that describes timing of an in-cylinder combustion parameter relative to piston position and is preferably measured with reference to crank angle. In-cylinder combustion parameters include a location of peak cylinder pressure (LPP) and a mass-burn fraction. A mass-burn fraction indicates a piston crank angle position at which a portion of the mass fraction of a cylinder charge is burned. A mass-burn fraction of interest includes a CA50 point (in crank angle degrees relative to TDC) at which an accumulated heat release of a cylinder charge reaches 50% of a total heat release. Combustion phasing is affected by temperature and composition of a cylinder charge during operation in an HCCI combustion mode.
HCCI control schemes achieve and maintain preferred combustion phasings by controlling temperature and composition of a cylinder charge using engine valve phasing and EGR flowrate controls. During a fast transient event from a low-load condition to a high-load condition, actual cylinder charge temperatures and compositions may deviate from preferred cylinder charge temperatures and compositions due to slow dynamic responses of an EGR valve and engine valve phasing control devices and associated fill-times. Such slow dynamic responses may result in over-advanced combustion phasing and objectionable audible combustion noise.
Systems for managing an increase in combustion noise during a low-to-high load transient include restricting a fuel ramping rate to avoid sudden and rapid deviation in the cylinder charge temperature and composition. Restricting a fuel ramping rate restricts a rate of increase in engine load, negatively affecting vehicle driveability.